1. Field of the Invention
The present invention relates to an apparatus and process for cleaning a workpiece with a cleaning medium maintained at a single fluid phase under conditions such that the workpiece is exposed to a single fluid phase of the cleaning medium. More particularly, the present invention relates to an apparatus and process for cleaning a workpiece with carbon dioxide and a co-solvent under conditions such that the workpiece is exposed to a single fluid phase of the carbon dioxide and co-solvent.
2. Description of the Prior Art
Fluid heated to above the critical temperature, i.e., the temperature above which a gas cannot be liquefied by an increase in pressure, is known as supercritical fluid. This fluid can move between the state of high density and that of low one without phase transition. Since the supercritical fluid can change density continuously, the slight change of temperature or pressure can manipulate the thermodynamic and transport properties of the fluid. Water fluid, as an example, changes the dielectric constant from about 78 at room temperature and atmospheric pressure to roughly 6 at 647° K (the critical temperature) and 220 atm. (the critical pressure). The character of water fluid changes from one that supports only ionic species to one that dissolves even paraffins and aromatics.
Due to this unique dielectric behavior property, numerous fundamental and applied research endeavors have been directed to reaction and separation processes that employ supercritical fluids, especially those that are associated with the environment. Supercritical fluids such as water and carbon dioxide are compatible with the earth's environment. Some applications and uses of supercritical fluids of carbon dioxide (SCFCO2) in processing solids and liquids are described in Chemical and Engineering News, June 1999, pages 11–13.
It has long been desirable to remove, in a precise and repeatable manner, organic, particulate and ionic contamination in developed resist films from components and assemblies without the use of water rinses or extensive post-cleaning drying. Carbon dioxide, either alone or in combination with other solvents, has been used to carry out such cleaning.
U.S. Pat. No. 5,377,705 describes a system for cleaning a workpiece with a multi-phase cleaning medium. However, when this apparatus is used to clean developed resist of sub 100 nm size (nano-images) in a multi-phase carbon dioxide, image collapse occurs. The liquid CO2 in the a multi-phase cleaning medium, being of higher surface tension than the supercritical phase, exerts an undesirable physical force on the developing image, thereby inducing image collapse.
U.S. Pat. No. 5,013,366 discloses a cleaning process using dense phase gases and phase shifting, i.e., shifting to and from the supercritical phase. In this process, carbon dioxide is the preferred dense phase gas, which may be mixed with co-solvents, such as anhydrous ammonia gas, and compressed to the supercritical fluid phase. This patent also discloses the use of carbon dioxide, co-solvents, and ultrasonic energy to enhance cleaning.
U.S. Pat. No. 5,068,040 discloses the excellent solvent/oxidant properties of supercritical ozone dissolved in liquid or supercritical carbon dioxide or water in dissolving and/or oxidizing inorganic materials. However, the presence of water presents problems with water recycling and disposal.
U.S. Pat. No. 2,617,719 discloses a process and apparatus for cleaning porous media, such as oil-bearing sandstone. The cleaning cell is supplied with a solvent and a dissolved gas, such as carbon dioxide. Used solvent is vented to the atmosphere. Solvent venting creates hazards to the environment that are unacceptable by today's standards.
Additional cleaning, extracting and stripping process are disclosed in U.S. Pat. Nos. 4,879,004; 5,011,542; 4,788,043 and 5,143,103.
The removal of selected portions of pattern films, as a form of semiconductor processing in forming high-resolution images, is a particularly useful application of a supercritical fluid. This is described in U.S. Pat. Nos. 4,944,837; 5,185,296 and 5,665,527.
Of particular concern is the inability to attain high aspect ratio images, i.e., height to width of image ratio. In general, aqueous based developers exert a high surface tension force, which causes images of <150 nm to fold inwardly. This problem has been described by Tanaka in Japanese J. Appl. Physics, vol. 32, pages 6059–6064 (1995). The image collapse is caused by the high surface tension of water (80 dynes/cm) exerting a physical force on the fragile lines/space patterns of resist. Thus, a lower surface tension developer would be advantageous to use.
Although a lower surface tension developer, such as heated water, has been described in U.S. Pat. No. 5,474,877, the surface tension of this system is still above 50 dynes/cm in the developer/rinse process.
Supercritical fluid of CO2 has been utilized as a resist developer. The use of supercritical fluid of CO2 is particularly advantageous in that the surface tension of SCFCO2 is less than 20 dynes/cm (see Jacobsen, J. Org. Chem., volume 64, pages 1207–1210 (1999)).
We have found that when the apparatus described in the previously cited U.S. Pat. No. 5,377,705 is used to develop resist in SCFCO2 of sub 100 nm size, i.e., nano-images, image collapse occurs. In the processing of the resist-coated wafer according to this patent, the developer chamber is pre filled with liquid CO2 and not SCFCO2. The liquid CO2 is then converted into SCFCO2 phase by heating to 31° C. and a 73.8 bar pressure. Being of higher surface tension, the liquid CO2 exerts an undesirable physical force on the developing image, thereby inducing image collapse.
It would be advantageous to introduce SCFCO2 having a lower surface tension into the process vessel for developing resist or for improved cleaning of wafers and reactive ion etch or other semiconductor process residues, such as those described in U.S. Pat. No. 5,908,510.